1. Technical Field
The present invention relates to a coaxial cable and a manufacturing method of the same, and particularly relates to a micro-coaxial cable using a metal nanoparticle paste sintered body in a conductive layer, and the manufacturing method of the same.
2. Description of Related Art
In a general coaxial cable, a low permeability insulating layer is formed on an outer periphery of an electric conductor positioned in a center. Further, a conductive layer is formed on an outer periphery of the insulating layer, by winding metal wires or a metal film around the outer periphery of the insulating layer. The conductive layer serves as a shield layer.
Meanwhile, in a method of forming the conductive layer by winding the metal wires or the metal film around the outer periphery of the insulating layer, it is difficult to manufacture a long coaxial cable, with a tendency of further micronizing a cable (for example, see patent document 1).
Here, in order to further micronize the cable, patent document 2 discloses a technique of coating a surface of the insulating layer made of fluororesin with a conductive nanoparticle paste, instead of winding the metal wires or the metal film around the outer periphery of the insulating layer, thereby providing a metal thin film as a shield layer made of a metal nanoparticle sintered body which is obtained by sintering metal nanoparticles by energization heating (for example see patent document 2).
Also, as a related technique, patent document 3 discloses a technique of improving bonding properties by making the fluororesin have various functional groups, and also patent document 4 discloses a technique in which by performing sintering by laser beams instead of sintering by energization heating when current wiring is formed on a substrate, only metal nanoparticles of a wiring part are sintered, and metal nanoparticles of the other part are removed. Further, non-patent document 1 describes a technique of decreasing a sintering temperature in a gold nanoparticle ink, in the sintering by the laser beams.    (Patent document 1)    Japanese Patent Laid Open Publication No. 06-187847    (Patent document 2)    Japanese Patent Laid Open Publication No. 2006-294528    (Patent document 3)    Japanese Patent Laid Open Publication No. 07-18035    (Patent document 4)    Japanese Patent Laid Open Publication No. 2006-38999    (Non-Patent document 1) Tae Y. Choi and Dimos Poulikakos, Applied Physics Letters, 85, p. 13-15,2004
However, there is a problem of easily causing external deformation such as bending to occur, when a force is added to a micro-cable which is formed by decreasing a wire diameter of the cable. Even in a case of a slight external deformation such as bending that occurs in the micro-cable, there is a problem that decline of adhesion occurs between an insulating layer in the cable and a shield layer made of a metal thin film. By such a low adhesion between the insulating layer and the metal thin film, there is also a problem that the metal thin film is peeled off from the insulating layer, thus causing a breakage of the metal thin film. As a result, conductivity of the shield layer, being a conductive layer, becomes insufficient, and shield characteristics are deteriorated.
Note that in order to improve the adhesion of the fluororesin, a method of applying chemical and physical etching to the fluororesin can be taken into consideration. However, this method involves a problem that a working process is complicated, a facility cost is increased, and a waste liquid is generated, thus posing a problem in terms of mass-production.
In the patent document 2, in order to further micronize a cable, the metal thin film made of the metal nanoparticle paste sintered body which is obtained by sintering the metal nanoparticles by energization heating, is formed on the insulating layer. However, when the sintering by energization heating is performed, heat treatment is applied not only to the nanoparticle paste to be sintered but also to the insulating layer, being its underlying layer. When the insulating layer is also heated, there is a possibility that decline of adhesion occurs between the insulating layer and the metal thin film, due to deformation and change of quality of the insulating layer. Therefore, even in a case of a slight external deformation such as bending that occurs in the micro-cable, there is a possibility that the metal thin film is peeled off from the insulating layer, thus causing the breakage of the metal thin film. As a result, there is a possibility that conductivity of the shield layer, being the conductive layer, becomes insufficient, and the shield characteristics are deteriorated.